1. Field of the Invention
The present invention is generally in the field of communications. More specifically, the present invention is in the field of modem communication over a telephone line.
2. Background Art
Some applications require pulse dialing compliance in communications devices, such as modems, that communicate over a telephone line. The pulse dialing circuits in modems typically cause large voltage spikes, or voltage peaks, on the telephone line when the modem is operating in a pulse dialing mode. Although protection devices such as metal oxide varistors (“MOV”) and sidactors in modems limit the voltage spike to 400.0 volts, some applications require the maximum voltage spike to be only 230.0 volts. To meet the requirement of a maximum voltage spike of 230.0 volts, an expensive external limiting circuit is often required.
FIG. 1 shows a block diagram including some of the circuit blocks in an exemplary modem for communication over a telephone line in a pulse dialing mode. Although only some of the blocks in the exemplary modem have been shown, the block diagram of FIG. 1 is referred to as “modem 100” in the present application for ease of reference. In modem 100 in FIG. 1, receiver 122 is connected to diode bridge 108 via tip and ring (“tip/ring” or “T/R”) line 114. AC driver 102 is coupled to T/R line 114 via line 116, and DC driver 104 is coupled to T/R line 114 via line 118. Clamp 106 is coupled to T/R line 114 via line 120. A telephone line is connected to diode bridge 108 at TIP terminal 110 and RING terminal 112.
Describing modem 100 in more detail, receiver 122 receives data over a telephone line via T/R line 114 and diode bridge 108. AC driver 102 sets the termination impedance of modem 100 and further includes a transmitter (not shown in FIG. 1) for sending data over a telephone line. DC driver 104 controls the DC loop current drawn from T/R line 114, and further includes a pulse dialing circuit (not shown in FIG. 1) for communicating over a telephone line through pulses generated by switching the DC loop current on and off.
Clamp 106 typically comprises a high voltage transistor to enable the clamp during pulse dialing mode and an MOV to prevent the size of the voltage spikes generated on T/R line 114 in pulse dialing mode from exceeding a predetermined limit. Diode bridge 108 rectifies the telephone line voltage at TIP terminal 110 and RING terminal 112 to provide voltage of the appropriate polarity to the circuitry in modem 100.
FIG. 2 shows a schematic diagram of DC driver 104 in modem 100 in FIG. 1. DC driver circuit 204 includes exemplary pulse dialing circuit 202, which comprises resistors 214, 218, 230, and 234, op amp 224, and switches 220, 238, and 242. In FIG. 2, a first terminal of resistor 284 is connected to node 206, also referred to as the tip and ring node or the T/R node, and a second terminal of resistor 284 is connected to node 208. A T/R line can be connected to node 206. T/R line at node 206 can be further connected to a telephone line via a diode bridge, such as diode bridge 108 in FIG. 1.
A first terminal of capacitor 210 is connected to node 208, and a second terminal of capacitor 210 is connected to ground. A first terminal of capacitor 212 is connected to node 208, and a second terminal of capacitor 212 is connected to ground. A first terminal of resistor 214 is connected to node 208, and a second terminal of resistor 214 is connected to node 216. A first terminal of resistor 218 is connected to node 216, and a second terminal of resistor 218 is connected to ground. A first terminal of switch 220 is connected to node 216, and a second terminal of switch 220 is connected to voltage source 222 (also referred to as “Vdcdac”). A “+” input terminal of op amp 224 is connected to node 208, and a “−” input terminal of op amp 224 is connected to node 228.
A “V+” terminal of op amp 224 is connected to voltage source 226 (i.e. “Vdd”), and a “V−” terminal of op amp 224 is connected to ground. The output of op amp 224 is connected to node 240 (also referred to as node EIO). A first terminal of switch 242 is connected to node 240, and a second terminal of switch 242 is connected to ground. A first terminal of resistor 230 is connected to node 228, and a second terminal of resistor 230 is connected to voltage source 232, i.e. “Vref”. A first terminal of resistor 234 is connected to node 228, and a second terminal of resistor 234 is connected to node 236 (also referred to as node EIF).
A first terminal of switch 238 is connected to node 236, and a second terminal of switch 238 is connected to ground. The base of transistor 244 is connected to node 240, and the collector of transistor 244 is connected to node 206. The base of transistor 246 is connected to the emitter of transistor 244, and the collector of transistor 246 is connected to node 206. The emitter of transistor 246 is connected to node 236. A first terminal of resistor 248 is connected to node 236, and a second terminal of resistor 248 is connected to ground.
Pulse dialing circuit 202 can operate in a normal mode and a pulse dialing mode. In the normal mode, pulse dialing circuit 202 uses op amp 224 and voltage source 222 to control how much DC loop current (also referred to as “IT”) is drawn from T/R line at node 206 by transistors 244 and 246. In pulse dialing mode, pulse dialing circuit 202 uses two states, an “off-hook” (also referred to as “make”) state and an “on-hook” (also referred to as “break”) state, to turn the DC loop current (i.e. “IT”) drawn by transistors 244 and 246 on and off.
At the initiation of a “break” state, switches 242 and 238 close, thereby shorting the base of transistor 244 and the emitter of transistor 246 to ground. As a result, transistors 244 and 246 shut off and “IT” immediately goes to zero. The rapid change in “IT” from a “make” current level to zero induces a voltage spike on T/R line at node 206 as a result of normal load inductance on T/R line at node 206. The resulting voltage spike on T/R line at node 206 requires a costly clamping circuit, such as clamp 106 in FIG. 1, to meet maximum voltage spike requirements in some applications. Additionally, the sharp transition that results from a rapid change in “IT”, i.e. the DC loop current, generates high frequency harmonics that may interfere with additional services, such as digital subscriber line (“DSL”), that may be sharing the same telephone line as a modem including exemplary pulse dialing circuit 202.
Thus, there is a need in the art for a pulse dialing circuit that limits the size of voltage peaks in pulse dialing mode to meet application requirements without the use of a costly limiting circuit. Additionally, there is a need in the art for a pulse dialing circuit that does not generate high frequency harmonics that interfere with various services, such as DSL services, sharing a telephone line.